Light waves propagating through an inhomogeneous space are subject to changes caused by interference, refraction, and diffraction. Thus, for example, light waves traveling along a path through a varying density gas and reflected by a mirror back along the same path to their source are likely very different than when initially radiated from the source. However, by replacing the mirror with a device that produces optical phase conjugate light waves, a different result is obtained. The optical phase conjugate light waves returning to the source along the same path followed by the light waves originally produced by the source are phase reversed, but are otherwise the same as when emitted by the source. Thus, optical phase conjugation appears to reverse time by "undoing" any changes in the light waves caused by passage through an inhomogeneous medium. Optical phase conjugation therefore compensates for the inhomogeneities of the intervening space between the source and the device.
One method of generating phase conjugate light waves employs stimulated Brillouin scattering. High-intensity coherent light emitted by a laser is directed at a cell filled with a gas, liquid, or transparent solid. The intense light causes periodic changes in the density of the material in the cell that alter the index of refraction of the material in a periodic pattern corresponding to the periodic density changes. These periodic density fluctuations in the material scatter the incident light, reflecting a portion of it. The reflected light interferes with the incident beam, causing further density variations in the medium. The cumulative effect of this process continues, eventually creating a "reflected" optical conjugate light wave that emerges from the cell in the opposite direction from that traveled by the incident light emitted by the laser. One disadvantage to this method for producing an optical phase conjugate wave is that a laser source producing over a million watts per square centimeter of intensity is required to initiate Brillouin scattering. A second disadvantage to this method is that the conjugate wave produced is slightly downshifted in frequency relative to the light from the source.
Fortunately, optical phase conjugate light waves can also be produced by an alternative method that does not require as powerful a light source. This alternative method is called four-wave mixing (FWM), because it involves the interference of four light waves of equal wavelength inside a nonlinear medium. One of the four light waves is referred to as a probe beam and corresponds to the light beam for which an optical phase conjugate light wave is desired. The optical phase conjugate light wave is the second of the four waves, and the remaining two light waves are called "pump waves." These two pump waves are directed toward each other from opposite sides into the nonlinear medium. Various types of nonlinear media may be employed, including simply a dye coating on a glass plate. Interference between the probe and pump waves within the nonlinear medium produces the phase conjugate light wave, which propagates along the same path as the probe wave, back toward the probe wave source.
Both the Brillouin scattering method and the conventional FWM methods produce optical phase conjugate light waves that traverse precisely the same path followed by the light emitted from the source. Those skilled in this art will recognize that there are a number of prospective applications for a system in which an optical phase conjugate light wave is produced that follows a different path from that of the probe light wave, particularly if the optical phase conjugate light wave can be steered non-mechanically through a desired deflection angle along a selected path. For example, the magnitude of the deflection angle could serve as an indication of the magnitude of a parameter that caused the deflection of the optical phase conjugate light wave.